1. Field of the Invention
The present invention relates to an optical beam scanning apparatus, which is used in a laser beam recording apparatus or the like.
2. Description of the Related Art
Conventionally, there has been proposed an optical beam scanning apparatus, which allows stable and rapid reading or recording of an image, realized by using a plurality of optical beams formed by an optical modulating apparatus providing a multi-frequency acoustic optical element (AOM) (see, for example, Japanese Patent Publication No. 63-5741, Japanese Patent Laid-Open Nos. 54-5455 and 57-41618, Japanese Patent Publication No. 53-9856, Japanese Utility Model Laid-Open No. 55-29414 and the like). In an optical beam scanning apparatus such as an optical beam recording apparatus or the like, in which an image is recorded using such an acoustic optical element, a plurality of oscillating circuits generates a plurality of high frequency signals having different frequencies. The generated high frequency signals are, in accordance with an image to be recorded, turned on or off so as to be mixed. The resultant signals are supplied to the AOM to drive it.
Accordingly, the AOM oscillates in accordance with the supplied high frequency signals and an optical beam incident on the AOM is diffracted in a direction corresponding to the frequency of the high frequency signals and by strength in accordance with the amplitude of the high frequency signals, so that a plurality of optical beams is emitted from the AOM. Scanning of the plurality of optical beams emitted from the AOM is carried out along a main scanning direction by means of an optical scanning system which comprises a rotating polygon mirror and the like, so that the optical beams are illuminated onto a recording material through a lens (scanning lens) which adjusts the scanned optical beams. A position in which an optical beam is illuminated is moved along a sub-scanning direction by conveying the recording material or by scanning an optical beam in the sub-scanning direction by means of a scanning means such as a galvanometer and the like. As a result, the optical beam illumination position moves two-dimensionally on the recording material so that an image is recorded thereon.
On the other hand, an optical beam which has passed through a scanning lens is displaced somewhat with respect to the position in which the optical beam is illuminated on the recording material due to an aberration of a scanning lens. The amount of displacement of the optical beam illumination position varies in accordance with the position in which the optical beam passes through the scanning lens. Accordingly, the line of motion of the position in which the optical beam scanned along the main scanning direction by means of an optical scanning system and illuminated on the recording material through the scanning lens curves with respect to the main scanning direction as illustrated in FIG. 12. Further, the optical beam represents different amounts and shapes of curvature in accordance with the position on the scanning lens in which the optical beam is scanned, i.e., the position in which the optical beam passes through (see FIG. 12).
Therefore, when an image is recorded by scanning a plurality of optical beams simultaneously, since scanning lines recorded by the respective optical beam are different from each other in their curved shapes, the intervals between scanning lines become non-uniform, thereby resulting in an unevenness in the image. Especially, when the intervals between scanning lines to be simultaneously recorded by a plurality of optical beams are widened to record the image by a so-called interlace, since the intervals between the positions on the scanning lens in which the respective optical beams are scanned become wide, these optical beams are greatly different from each other in their amounts and shapes of curvature. As a result, unevenness of the interval between the scanning lines to be recorded by the respective optical beams appears remarkably and periodically.
The unevenness of the interval of scanning lines especially becomes a problem when an image represented with dots which are very often employed in the field of printing is recorded. Namely, since the area of the dots represent image density, the dots must be recorded such that the area thereof accurately corresponds to the density. However, when there is the unevenness of the interval of scanning lines as explained above, the dot area varies so that the recorded image may be viewed as that which is different from the original one. Also, when the unevenness is periodically generated, there is an inconvenience in which the unevenness is excessively represented depending upon a relationship with respect to the interval of dots. Further, recently in the field of printing, an image is recorded in high density by 600 to 700 scanning lines per inch. In this way, when an image is recorded at high density, the intervals between the scanning lines need to be made strictly uniform.
In addition, when an image is recorded by scanning one optical beam only in a main scanning direction and by conveying a recording material to carry out sub-scanning, since the optical beam scans a fixed position on a scanning lens, each scanning line has a uniform amount and shape of curvature so that the intervals between the scanning lines are substantially uniform. However, when an image is recorded in high density on, e.g., a microfilm and the like, using the above-described optical beam recording apparatus, the image recorded on a microfilm may be enlarged by high magnification. In this case, even though the intervals between scanning lines are uniform, if a scanning line curves only by 0.1 .mu.m, this curved portion is viewed as image distortion.
On the other hand, with relation to the above description, it has been known that an oscillator circuit which generates a signal to be inputted to an AOM is comprised of a voltage controlled oscillator (VCO) having a variable oscillating frequency in accordance with an applied voltage (Visibility and Correction of Periodic Interference Structures in Line-by-Line Recorded Images, Journal of Applied Photographic Engineering Volume 2, Number 2, Spring 1976). However, this technique disclosed in the above-described publication employs the VCO in order to correct an error with respect to a preset frequency and does not change tile frequency of the VCO during scanning of optical beams.